Mary Keitany was one of the world’s fastest long-distance runners until she had her first child. Then she became the fastest half-marathoner on Earth.

“I need to run well to give a good future to my son,” says Keitany, 29, who broke the half-marathon world record in February, 15 months after giving birth. “To run you need big motivation — the motivation to change your life — and since I became a mother I have more responsibility. Kenya is a poor country, and when I compete I am ready to run fast.”

In the world of female distance running, giving birth may actually give runners a competitive edge. While there are many physiological factors — including an increase in flexibility and blood volume — the real benefits most likely occur in the mind.

“The amount of blood that’s pumped seems to elevate longer, but that’s not during a half-marathon, that’s just sitting around,” Dr. James Pivarnik, director of Michigan State University’s centre for physical activity and health, says. “Physiologically, if there’s any advantage, it’s probably done in two months. My suspicion is they’re feeling a sense of relief or fulfillment. It’s not physiological as much as, ‘I’ll show them.’ ”

Chris Lundgren is the author of Running & Pregnancy, and her marathon times improved each time after she gave birth. “In the back of your mind when you’re running a marathon is: ‘This is nothing compared to labour.’ You have a totally different benchmark for pain,” says Lundgren, 46, who reported in 2003 that 785,000 U.S. women were either running while pregnant or after having just given birth — a number which has certainly increased today. “Pregnancy running is a bridge between your life as a runner and your life as a running mom, and it has an endorphin buzz that’s much better than a cup of coffee or a glass of wine.”

One runner who has felt the effects of the pregnant-mom market is Cindy Lynch, who started the company Running Skirts with her twin sister in 2005, and last March launched a line for expecting mothers.

“After my first pregnancy, I had all my best times without as much training,” says the San Diego-based Lynch, 38-weeks pregnant and still running four miles a day.

“I call it ‘Magical Baby Juice,’ and when women see me running, they think it’s awesome. There was just so much bad information out there about pregnancy and running before.”

There are many examples of women doing extraordinary physical feats after childbirth. Olympic gold-medal winner Paula Radcliffe won the 2007 New York City marathon 10 months after having her child. And swimmer Dara Torres won three silver medals at the 2008 Olympics after giving birth — and she did it at 41 years old. However, even though Dr. Pivarnik says these women are outliers and that most new mothers — the ones who don’t make it into newspaper stories — will need all their energy just getting their babies into booties, one of Canada’s strongest distance runners is hoping to follow in Mary Keitany’s running shoes.

“I kept saying I’d wait, but emotionally, I couldn’t let go of this biological feeling,” says Tara Quinn-Smith, winner of the Montreal half-marathon in 2008 and 2009. Twenty-seven-weeks pregnant and hoping to run the marathon for Canada in the 2012 Olympics, Quinn-Smith, 31, says that if her pregnancy makes her into a better runner, it will be because it has given her a better life.

“Running is great, but if you don’t have other things, your life can fall out of balance,” she says. “For me, I think having a little one is definitely going to make me run better. It makes you appreciate so many more things.”

Can having a child increase your split times? Send an email to bkaplan@nationalpost.com and join the conversation online.

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